1. Field of the Invention
The present invention relates to a method of producing an extrusion-laminated member. Particularly, the invention relates to a method of stably forming a film of a molten resin by extrusion lamination. More particularly, the invention relates to a method forming a film suppressing the swinging of film and excessive necking during the extrusion lamination featuring an increase in the width of film on flat portions, an increase in the yield of the product and an improvement in the adhesion to the base member.
2. Prior Art
It is a widely accepted practice to coat the surfaces of a metal with a resin layer as means for imparting corrosion resistance to a metal material. The coating method employed by this technology can be represented by a method of coating the surfaces of a metal with a thermosetting resin such as epoxy resin, phenol resin, acrylic resin or polyester resin dispersed in a solvent, a method of sticking a film that has been formed in advance such as of polyester, olefinic resin or polyamide onto a metallic base member via an adhesive such as of isocyanate, epoxy or phenol, or the like method.
There has further been widely known to stick a thermoplastic resin onto a metallic base member by utilizing the heat-melt-adhering property of the thermoplastic resin. This method can be represented by a method of heat-adhering a film that has been formed in advance such as of a thermoplastic polyester onto a metal plate, or a method of sticking a thin molten film such as of a thermoplastic polyester resin that has been extruded onto a metal plate.
U.S. Pat. No. 5,407,702 (International Patent Publication No. 503378/1999) discloses a method of coating both surfaces of a metal strip while extruding a resin to form a film. According to this method, a metal strip such as of an aluminum alloy is passed through a pre-conditioner, two extrusion dies, a post-heating machine and a cooling system, thereby to coat both surfaces of the strip with a thin polyester film. According to the apparatus shown in FIG. 1 of the specification of this U.S. patent, thin polyester films extruded by the dies are spread by first rolls (drawing rolls), cooled by second rolls (contact rolls), and are press-adhered onto the heated metal strip by third rolls (laminating rolls).
The present inventors has proposed in Japanese Unexamined Patent Publication (Kokai) No. 138315/1998 a method of forming a resin coating on at least one surface of a metallic base member, i.e., a method of producing a resin/metal laminated member by arranging a metallic base member-heating zone, a die for feeding a thermoplastic resin like a film, a pair of hot laminating rolls for adhering the thermoplastic resin onto at least one surface of the metallic base member, and quick-quenching means for quickly quenching the formed laminated member along a passage of the metallic base member, conveying a molten film of the thermoplastic resin from the die by using the pair of hot laminating rolls to feed the film to a nipping position between the hot laminating rolls, and melt-adhering a thin film of the thermoplastic resin onto at least one surface of the heated metallic base member using the hot laminating rolls.
Further, Japanese Unexamined Patent Publication (Kokai) No. 100006/1999 discloses hot laminating rolls for extrusion-laminating a molten film of a thermoplastic resin that is extruded onto the surface of a metallic base member, wherein the roll of a side contacting to the molten film of the resin is an elastic roll, a portion for press-adhering the molten film of resin onto the metallic base member is heated at a temperature (T1) which is not lower than 50xc2x0 C. but is lower, by 30xc2x0 C., than a melting point of the thermoplastic resin, and other portions are heated at a temperature (T2) lower than the temperature (T1).
In conducting the extrusion lamination, in general, an air gap is formed between a lip portion of the T-die and a nipping portion of the laminating rolls. In the conventional extrusion lamination using a pair of laminating rolls, a limitation is imposed on decreasing the air gap due to mechanical limitation such as that a base member is fed to the nipping position of the laminating rolls in addition to feeding the molten resin film.
When the air gap increases in conducting the extrusion-lamination of a general-purpose resin having poor property of being formed into a film, both ends of the film tend to swing, i.e., selvages tend to swing toward the right and the left making it difficult to form the film. Or, even though the film could be formed by decreasing the speed of formation, the demand for forming the film at high speeds is not fulfilled.
Due to the shape memory property of the molten resin, further, there occurs excess of necking causing the width of the molten resin film to become narrow, causing the edge beads of the resin film to increase, causing the flat portion of the film to be contracted, and causing an increase in the loss of the resin.
According to the three-roll stacking system quoted above, the air gap can be decreased, but this system is not still fully satisfactory from the standpoint of producing a resin/metal laminated member of high performance maintaining high productivity. The above-mentioned prior art requires the operation for heating a metal plate prior to laminating a resin layer such as of polyester, and the operation for heating the resin/metal laminated member after the resin layer has been laminated to complete the melt-adhesion. However, the operations for heating, many times, the metal plate and the resin-coated metal plate at a temperature higher than the melting point of the polyester, cause deterioration due to heat-softening of the metal plate, heat decomposition and heat oxidation of the resin, resulting in a drop in the properties of the laminated member. The drop in the properties becomes conspicuous as the number of times of heating increases or, generally, as the thickness of the thin resin film becomes small.
In producing a resin/metal laminated member for cans, further, there remains a technical assignment in that a thin resin film must be strongly adhered maintaining a uniform thickness onto the metal plate. In the case of a film which has been biaxially stretched in advance, for example, the film can be laminated by heat-adhesion maintaining a relatively uniform thickness but must be stretched through a separate step; i.e., complex steps are required. On the other hand, the three-roll stacking system quoted above requires a cumbersome operation of cooling and forming a film while stretching the extruded molten resin into a thin film arousing such a problem that the temperature on the surface of the resin drops when it is being formed into a film making it difficult to accomplish a strong heat adhesion onto the metal plate. Besides, the resin film develops wrinkles on the third roll as the thickness of the film decreases and as the speed increases.
It is therefore an object of the present invention to provide a method of stably forming a film of a molten resin by extrusion lamination, preventing the swinging of the film and excess of necking during the extrusion lamination, increasing a width of film in the flat portion, increasing the yield of product and improving the intimate adhesion onto the base member.
Another object of the present invention is to provide a method of producing a resin/metal laminated member preventing the heat softening of the metal, ignition loss and heat oxidation of the resin as much as possible, featuring uniformity and small thickness, and excellent adhesion to the metal, maintaining a high productivity and a high production yield.
A further object of the present invention is to provide a method of producing a resin/metal laminated member useful for the cans, the resin/metal laminate that is formed being capable of withstanding such severe workings as deep-draw forming, bend-elongation working and ironing working, the formed product after worked exhibiting excellent corrosion resistance.
According to the present invention, there is provided a method of producing an extrusion-laminated member by extruding a thermoplastic resin from a T-die into the form of a film and laminating the film on a metallic base member using laminating rolls, wherein a pre-roll is disposed between the T-die and the laminating roll to receive the film on the side opposite to the side that adheres onto the base member over the whole width of the film, the film received by the pre-roll is fed to the laminating rolls, conveyed by being supported by the laminating rolls on the side opposite to the side that adheres onto the base member and is fed to a nipping position so as to be melt-adhered onto the heated metal blank.
According to the production method of the present invention, it is desired that:
1. The pre-roll is so disposed that the angle (xcex8) of the resin film wound on the laminating roll is from 2 to 45 degrees when the resin film received by the pre-roll is fed to the laminating rolls and is conveyed by being supported thereby;
2. The film extruded from the T-die is supported so as to be wound on the pre-roll at an angle (xcex1) of not smaller than 10 degrees and is fed to the laminating rolls;
3. The pre-roll is disposed to rotate independently of the laminating rolls, the peripheral speed of the pre-roll being not smaller than 0.1 times but not larger than 1.0 time of the peripheral speed of the laminating rolls;
4. The pre-roll has a surface shape for reliably holding the selvages at portions corresponding to the selvages of the film;
5. The surface shape is steps or a coarse surface;
6. The pre-roll is a crown roll having a curvature;
7. The film is maintained at a temperature higher than a glass transition point (Tg) of the thermoplastic resin as measured at a position separated away from the pre-roll and on the side that adheres onto the base member;
8. The base member is fed in a direction nearly at right angles with a line connecting the centers of the pair of laminating rolls; and
9. A pair of T-dies and a pair of pre-rolls are disposed nearly symmetrically with respect to the base member to laminate the resin films on both surfaces of the base member.